of Gold and other Metals to Light. 517 
the fluid may be removed by a siphon; being now allowed to 
stand for a week, a second deposit will be produced ; if the fluid 
be again removed and allowed to stand for some months, another 
deposit will be obtained, and the fluid will probably be of a 
bright ruby ; if it be now allowed to stand for several months, it 
will still yield a deposit, looking however more like a ruby fluid 
than a collection of fine particles at the bottom of the fluid, 
whilst traces of yet finer particles of gold in suspension may be 
obtained by the lens. All these deposits may be washed with 
water and will settle again; the coarser are not much affected, 
but the finer are, and tend to aggregate ; nevertheless specimens 
often occur, especially after boiling, which tend to preserve their 
fine character after washing, if the water be very clean and pure. 
The colour of these particles whilst under, or diffused through 
water, is, by common reflected light, brown, paler and richer, 
sometimes tending to yellow, and sometimes to red. The same 
difference is shown when illuminated by sunlight. Everything 
tends to show that the light reflected is very bright considering 
the size of the particles, and therefore of the reflecting surfaces ; 
yet comparing by the cone of light a ruby fluid when first pre- 
pared and before it has become very sensibly turbid, with the 
same fluid after the evident turbidity is produced, in both of which 
cases I believe the gold to be in solid metallic particles, though 
of different sizes, it would seem that more light is transmitted 
and absorbed and less reflected by the finer particles than by the 
coarser set, the same quantity of gold being in the same space. 
I believe that there may be particles so fine as to reflect very 
little light indeed, that function being almost gone. Occasion- 
ally some of the fluids containing the very finest particles in sus- 
pension, when illuminated by the sun’s rays and a lens, appeared 
to give a fine green reflexion, but whether this is a true colour 
as compared to white light, or only the effect of contrast with 
the bright ruby in the other parts of the fluid, I am not pre- 
pared to say. 
When the deposits were examined in the dark tube by trans- 
mitted light, bemg first diffused in more or less water to give them 
the form of fluid, those first deposited, and therefore presumed 
to be the heavier and larger, transmitted a pure blue light. 
The second and the third had the same character, perhaps the 
fourth, if the subdivision into portions had been numerous ; 
then came some which transmitted an amethystine ray from the 
white of paper; and others followed progressing to the finest, 
which transmitted a rich ruby tint. It is probable that many of 
these deposits were mixtures of particles having different cha- 
racters, and this is perhaps the reason that in some cases, when 
the fluids were contained in round-bottomed flasks, the lens-like 
